Bedford Borough Strategic Flood Risk Assessment Level 1
Bedford Borough Strategic Flood Risk Assessment – Level 1
Notice This report was produced by Atkins Limited for Bedford Borough Council for the specific purpose of the Bedford Borough Strategic Flood Risk Assessment. This report may not be used by any person other than Bedford Borough Council without Bedford Borough Council’s express permission. In any event, Atkins accepts no liability for any costs, liabilities or losses arising as a result of the use of or reliance upon the contents of this report by any person other than Bedford Borough Council. Atkins Limited Document History
JOB NUMBER AND REFERENCE: DOCUMENT REF: Bedford SFRA v1.4 - Level 1.doc 5061078/72/DG/054
1.4 FINAL CN/JL KP TR CN July 2008
1.3 Final for Review CN KP TR CN June 2008
1.2 Final for Review JL KP TR JL May 2008
1.1 Final for Client Review JL KP TR JL May 2008
1 Draft for Client Review TG, JL KP TR, SF JL March 2008
Revision Purpose Description Originated Checked Reviewed Authorised Date
5061078/Bedford SFRA v1.4 - Level 1.doc
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Bedford Borough Strategic Flood Risk Assessment
Contents Section Page Glossary of Terms 3 Non Technical Summary 5 1. Introduction 7 1.1 Need for a Strategic Flood Risk Assessment 7 1.2 Scope of the Document 7 2. Planning Policy Relating to Flood Risk 9 2.1 National Planning Policy 9 2.2 Local Planning Policy Relating to Flood Risk 11 3. Data Collection and Review 15 3.1 Consultation 15 3.2 Catchment Characteristics 16 3.3 Sources of Flood Risk Data 16 3.4 Data Summary 24 4. Flood Risk 27 4.1 Sources of Flooding 27 4.2 Flood Defences 30 4.3 Flood Risk Sensitivity 30 5. Developer Guidance 35 5.1 Generic Developer Guidance 35 5.2 Site Specific Information for Developers 41 6. Conclusions and Recommendations 45 6.1 Conclusions 45 6.2 Recommendations for the Level 2 SFRA 46 References 49
List of Tables Table 2.1 – Bedford Borough Local Plan and Local Policies: Policies relating to flood risk 11 Table 2.2 – East of England Plan: Policy SS14 Development and Flood Risk 12 Table 2.3 – East of England Plan: Policy WAT 4 Flood Risk Management 12 Table 2.4 - MKSMSRS: Strategic Policy 3 Sustainable Communities 13 Table 3.1 –Hydraulic Model Summary 18 Table 3.2 – Anglian Water’s DG5 register: Key locations and type of sewer 18 Table 3.3 - Flood Risk Vulnerability Classification (based on Table D2 of PPS25) 21 Table 3.4 – Appropriate Development for each Flood Zone (based on Table D3 of PPS25) 22 Table 4.1 – SUDS Options that may be utilised within Bedford Borough 31 Table 6.1 – Recommended further hydraulic modelling 46
Appendices Appendix A Bedford Borough Mapping
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A.1 Location Overview A.2 Development Allocations – excluding allocations within the TCAAP A.3 Flood Risk Overview – Environment A.4 Assets and Structures Overview A.5 Flood Warning Overview A.6 Bedford IDB Catchment A.7 Detailed Mapping – Index Map A.8 Detailed Flood Risk Mapping A.9 Detailed Assets and Structures Mapping Appendix B B.1 Anglian Water DG5 Register B.2 IDB Flood Defence Structures (Summary Table)
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Glossary of Terms
Term Meaning / Definition AAD Average Annual Damages AEP Annual Exceedance Probability AFFMS Anglian Flow Forecasting Modelling System CFMP Catchment Flood Management Plan CSRIP Core Strategy & Rural Issues Plan DG5 Water companies record of Sewer Flooding LDDs Local Development Documents LDF Local Development Framework LPA Local Planning Authority FWD Flood Warnings Direct FRA Flood Risk Assessment FSR Flood Studies Report GIS Geographical Information System Main River This term is used for watercourses shown on statutory maps held by the Environment Agency and DEFRA. They can include any structure or appliance for controlling or regulating the flow of water into, in or out of the channel. The Environment Agency has permissive powers to carry out works of maintenance and improvement on these watercourses (Main Rivers). MKSMSRS Milton Keynes and South Midlands Sub-Regional Strategy NFCDD National Flood and Coastal Defence Database PPS1 Planning Policy Statement 1: Delivering Sustainable Development PPS3 Planning Policy Statement 3: Housing PPS25 Planning Policy Statement 25: Development and Flood Risk RFRA Regional Flood Risk Assessment RPB Regional Planning Boards RSS Regional Spatial Strategy SFRA Strategic Flood Risk Assessment SMD Soil Moisture Deficit SOP Standard of Protection SUDs Sustainable Drainage Systems TCAAP Bedford Town Centre Area Action Plan
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Annual Exceedance Probability (AEP) The severity of the events discussed in this document are defined as Annual Exceedance Probabilities (AEP), the table below provides a summary of AEP and corresponding Return Periods. The AEP is the probability that there will be an event exceeding a particular severity in any one year. The Return Period is the average duration (in years) between events of a particular severity.
Annual Exceedance Probability Return Period 50% 1 in 2yrs 10% 1 in 10yrs 4% 1 in 25yrs 3.3% 1 in 30yrs 2% 1 in 50yrs 1.33% 1 in 75yrs 1% 1 in 100yrs 0.5% 1 in 200yrs 0.4% 1 in 250yrs 0.1% 1 in 1000yrs
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Non Technical Summary
This Level 1 Strategic Flood Risk Assessment (SFRA) has been produced for the Bedford Borough Council. It compliments the Bedford Town SFRA completed by WSP in 2006, and should be read in conjunction with this document. An SFRA is required under Planning Policy Statement 25: Development and Flood Risk, to facilitate sustainable planning and development on a regional scale. In producing this document, consultation with a number of parties has taken place. The key consultees utilised in the completion of this Level 1 SFRA have been the Environment Agency, Anglian Water, Bedford Borough Council and the Bedford Group of Internal Drainage Boards (IDB). The data collected during this Level 1 SFRA has resulted in a high level of understanding of flood risk within the Bedford Borough. The main source of flood risk within the Bedford Borough is fluvial flooding from the River Gt. Ouse, which runs from west to east across the centre of the Bedford Borough. There are also areas of flood risk within the north of the Bedford Borough from the River Til, Riseley Brook and Pertenhall Brook, all of which are main rivers maintained by the Environment Agency. To the south-east of Bedford Borough lie many watercourses maintained by the IDB, all of which have the potential to cause fluvial flood risk. Some locations within the Bedford Borough are also at risk from surface water and foul water flooding. There is no evidence of groundwater flooding or flooding from the many balancing ponds that are located within the Bedford Borough, however the IDB and the Environment Agency have identified potential areas at risk from groundwater flooding which will be further investigated in the Level 2 SFRA. There are no British Waterway owned canals within the Bedford Borough; however there are plans to develop a canalised section of waterway between Milton Keynes and Bedford. These proposals will be assessed in more detail during the Level 2 SFRA. Bedford Borough Council has already allocated a series of development sites within the Borough. These sites have been subject to site specific FRAs and have been committed to development. This Level 1 SFRA does not focus on already allocated sites, and further information on these sites can be found in the site specific FRAs that have been completed for them. This Level 1 SFRA, has highlighted the areas that are at risk of flooding within Bedford Borough. The ‘Key Service Centres’ that Bedford Borough have identified for future development, have also been assessed in terms of flood risk. In the development of this Level 1 SFRA, a series of assumptions have been developed. These assumptions were developed in conjunction with Bedford Borough Council, the Environment Agency and the Bedford Group of Drainage Boards and are listed below: • all areas at risk of flooding are undefended; • the Functional Floodplain has been classed as any area lying within Flood Zone 3; and • the climate change outline is taken to be the same as the current Flood Zone 2 outline. The outcome of this Level 1 SFRA has enabled a series of recommendations to be developed, which upon approval of this Level 1 SFRA will form a detailed proposal and methodology for the Level 2 SFRA.
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1. Introduction 1.1 Need for a Strategic Flood Risk Assessment Planning Policy Statement 25 Development and Flood Risk (Refs. 1, 2) outlines the principles by which the government seeks to reduce the risk of flooding to new developments. This is achieved through a variety of tools, primarily based on the site specific Flood Risk Assessment (FRA) for proposed developments. To facilitate catchment-wide development, larger scale FRAs are required. These are in the form of Regional Flood Risk Assessments (RFRAs) and Strategic Flood Risk Assessments (SFRAs). An SFRA provides the information necessary to perform the Sequential Test for a region. It will govern where development may take place now, and in the future. It aims to achieve these levels of development in a sustainable fashion. 1.2 Scope of the Document This Level 1 SFRA is required for Bedford Borough. A detailed SFRA has already been completed by WSP in 2006 (Ref. 3) for Bedford Town Centre; therefore this area is not included within the scope of this assessment. This document examines the planning policy relating to development and flood risk on both the national, regional and local scale. At a local scale this would also include the Surface Waters Plan (Ref. 13) for Marston Vale and is detailed further in Section 2.2. Sources of flood risk data are collated and reviewed to develop a catchment-wide understanding. A summary of the data collected and a review of this data is provided. This forms the basis for the assessment of flood risk in the Bedford Borough. This SFRA addresses potential climate change issues and sustainable methods of mitigating flood risk. SFRAs should be considered to be living documents, subject to regular review in response to changing policy requirements and improved understanding of flood risk in the areas to which they apply. This Level 1 SFRA will form the basis for the Level 2 SFRA. The Level 2 SFRA will provide detailed results which can be used in conjunction with the Level 1 SFRA.
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2. Planning Policy Relating to Flood Risk
This section covers a range of planning policy documents related to flood risk, including national, regional and local policy relevant to the Bedford Borough. PPS25 provides the key policy focus and is given due attention. Regional and local policy in the form of development plans and other similar documents support this national guidance. 2.1 National Planning Policy National Planning Policy plays a key role in shaping the direction in which Regional Planning Boards (RPB) and Local Planning Authorities (LPA) prepare their Regional Spatial Strategies (RSS) and Local Development Frameworks (LDF). Planning Policy Statements set out the Government’s national policies on different aspects of land use planning in England. The key Planning Policy Statement which has been instrumental in bringing forward SFRAs is Planning Policy Statement 25: Development and Flood Risk (PPS25). Other key statements which have influenced the scope of this SFRA include PPS11 and PPS32. The key principles promoted by these statements are described in the following sections. 2.1.1 PPS1: Delivering Sustainable Development PPS1 sets out the Government’s aims and objectives for delivering sustainable development, for current and future generations. One of the key principles set out in PPS1 is to ensure that sustainability is considered for the life time of new development by taking due account of the physical environment and the impacts of climate change. The key to delivering sustainable development is centred at the planning and design stages. PPS1 encourages LPAs to consider all aspects of the physical environment when identifying land for development. In particular, when preparing development plans, LPAs should identify the potential impacts that natural hazards may pose to new development and as far as possible, avoid development in areas at risk of flooding and sea level rise. Should development in areas of flood risk be required to meet the wider objectives of sustainable development, PPS1 supports the design of new development which accommodates natural hazards and the impacts of climate change to ensure the development is safe, sustainable, durable and adaptable. 2.1.2 PPS3: Housing PPS3 sets out the national planning policy framework for delivering the Government’s housing objectives. The policies set out in PPS3 should be taken into account by LPAs and RPBs in the preparation of the Local Development Documents (LDDs) and RSSs. PPS3 encourages LPAs to take account of the constraints of the physical environment and natural hazards, such as flooding, when identifying broad locations for housing development. PPS3 also states that a key objective of LPAs should be to continue to make effective use of land by re-using sites that have been previously developed. In addition the policy states that the national annual target is that at least 60% of new housing is provided on previously developed land. However, the policy also recognises that LPAs and RPBs will need to consider sustainability issues for some sites as they may not be suitable for housing. A key example of where sustainability of previously developed land may need further consideration is where land is vulnerable to flood risk.
1 Planning Policy Statement 1: Delivering Sustainable Development. Department for Communities and Local Government (December 2006)
2 Planning Policy Statement 3: Housing. Department for Communities and Local Government (December 2006)
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2.1.3 Background to PPS25 PPS25 sets out the Government’s policies for development and flood risk. The statement was released in December 2006 and replaces the former Planning Policy Guidance Note 25 (PPG25)1. The aims of PPS25 are to ensure that flood risk is taken into account at all stages in the planning process to avoid inappropriate development in areas at risk of flooding, and to direct development away from areas at highest risk. Where new development is unavoidable in such areas, policy aims to make it safe without increasing flood risk elsewhere and where possible, reducing flood risk overall. PPS25 follows the same guiding principles as PPG25, with notable additions including; • A more strategic planning approach to managing flood risk; • Stronger guidance on Flood Risk Assessments, at all stages of the planning hierarchy; • A clarified Sequential Test; • A new Exception Test, to account for instances where large developed areas have extensive areas within Flood Zones 2 and 3 and where a blanket ban on development would cause extensive social and economic blight; and • Clearer guidance on how to assess the impacts of climate change. The Planning and Compulsory Purchase Act 2004 requires that a Sustainability Appraisal is undertaken for RSSs’, Development Plan Documents (DPDs) and Supplementary Planning Documents. RPBs and LPAs are required under PPS25 to prepare and to implement planning strategies that help deliver sustainable development. In developing their policies and strategies, RPBs and LPAs should work with the Environment Agency and other relevant operating authorities and stakeholders in appraising, managing and reducing flood risk. As part of this process, RPBs should prepare RFRAs and LPAs should prepare SFRAs as freestanding assessments to contribute to the Sustainability Appraisal of their plans. Aspects of the SFRA will then feed in to more detailed site specific FRAs. PPS25 compliments other national planning policies and should be read in conjunction with Government policies for flood risk and water management, including Making Space for Water and the Water Framework Directive. 2.1.4 Sequential Test Developments should be directed to sites at the lowest probability of flooding from all sources, as indicated by the SFRA. To achieve this, the SFRA assesses land allocations and development control policies in terms of potential sources and probability of flooding, and the impact of climate change. 2.1.5 Exception Test The Exception Test will be informed by an SFRA, so that it may be demonstrated that the development provides wider sustainability benefits to the community that outweigh flood risk. The SFRA will form the basis for the development allocation and control processes. It should be noted that the Exception Test will only be used in exceptional circumstances and will only consider certain types of development in different flood risk areas, as detailed in Table D.3 of PPS25 (Ref. 1).
1 Planning Policy Guidance Note 25: Development and Flood Risk. DETR (2001)
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2.2 Local Planning Policy Relating to Flood Risk
2.2.1 The Bedford Borough Local Plan and Local Policies The Bedford Borough Local Plan (Ref. 3) and Core Strategy & Rural Issues Plan (CSRIP) outline a series of policies related to the development of the region. Of these policies, those that relate to flood risk and associated issues are detailed below in Table 2.1.
Policy Context Objectives Local Flooding The Borough Council will not permit development where: Plan NE16 • it would intensify the risk of flooding; or • it would be at an unacceptable risk from flooding; or • it would prejudice existing flood defences or interfere with the ability to carry out flood control and maintenance work; or • it would adversely affect wildlife habitat in the floodplain unless, the Borough Council, in consultation with the Environment Agency and Internal Drainage Board as appropriate, is satisfied that the developer will provide appropriate mitigation, protection and compensatory measures. Local Protection The Borough Council will seek to protect, and where possible, enhance, the water Plan and resources in the Borough by: NE24 Enhancement • not permitting developments which would adversely affect the quality or quantity of Water of water resources or their amenity or nature conservation value; Resources • not permitting development which would unduly restrict access to the river and other water bodies with recreational potential; • actively negotiating with developers in order to achieve more sustainable methods of surface water management and drainage. Local Floodplains The Borough Council will not permit development that compromises the capacity of the Plan U2 floodplain, balancing ponds, drainage pipes, channels and other flood defences and works to alleviate flooding, or would be at risk of being flooded, or would unacceptably increase the risk of flooding or pollution through seepage or run off. CSRIP Climate • The council will require development to: Change and CP26 • minimise the emission of pollutants into the wider environment; Pollution • have regard to cumulative impacts of development proposals on air quality, in particular in relation to air quality management areas; • minimise the consumption and use of energy, including fossil fuels, by design and choice of materials; • unless it can be demonstrated that – having regard to the type of development involved and its design - these requirements are not feasible or viable, achieve a minimum 10% reduction in carbon emissions (below the normal requirement set by the Building Regulations) in all new residential developments and above a threshold of 500m2 in new non-residential developments by measures which shall include, in new developments above a threshold of 1000m2 or 50 dwellings, the supply of at least 10% of the energy consumed in the new development to be provided from decentralised and renewable or low-carbon energy sources; • as a minimum, meet the national standards for building performance set by the current Building Regulations. Through the Allocations and Designations DPD process the Council may identify local development or site specific opportunities which justify the adoption and application of higher standards of building performance as set out in the Code for Sustainable Homes. Such higher standards may also be required by the Council where justified by changes in national guidance; • utilise sustainable construction techniques; • incorporate facilities to minimise the use of water and waste; • limit any adverse effects on water quality, reduce water consumption and minimise the risk of flooding; and • developers will be expected to submit a sustainability statement and energy audit with proposals for development. Table 2.1 – Bedford Borough Local Plan and Local Policies: Policies relating to flood risk
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2.2.2 East of England Plan (Draft) The East of England Plan constitutes the RSS for the East of England, which is comprised of Norfolk, Suffolk, Cambridgeshire, Essex, Hertfordshire and Bedfordshire. The RSS aims to contribute towards the achievement of sustainable development through a series of objectives, or policies. The document covers the period until 2021, but is considerate of the longer term. The East of England Plan that was consulted during this study was in draft format and has subsequently been published in May 2008. Of the many policies outlined in the document Policy SS14 and Policy WAT 4 are of primary importance in terms of flood risk and are outlined below in Table 2.2 and Table 2.3.
Policy SS14: Local development documents will: development and flood risk
Coastal and river - promote the use of Strategic Flood Risk Assessments to guide development away from flood risk is a floodplains, areas at risk or likely to be at risk in future from flooding, or where development significant factor in the would increase the risk of flooding elsewhere East of the England. - include policies to protect floodplains and land liable to tidal or coastal flooding from The priority is to development, based on the Environment Agency's Flood Zone maps, supplemented where defend existing necessary by historical and modelled flood data (e.g. Section 105 maps) and indications as to properties from other areas which could be at risk in future (including proposals for ‘managed retreat’ where flooding, and where appropriate) possible locate new development in - require that all developments and, where subject to planning control, all land uses (including locations with little or agricultural activities and changes to drainage in existing settlements) should not add to the no risk of flooding. risk of flooding elsewhere and should reduce flooding pressures by using appropriate sustainable drainage systems
- only propose development in floodplains, areas at flood risk or at risk of flooding in future, or where development would increase the risk of flooding elsewhere, where land at lower risk of flooding is not available, where there is a significant overriding need for the development, and the risk can be fully mitigated by design or engineering measures. Table 2.2 – East of England Plan: Policy SS14 Development and Flood Risk
Policy WAT 4: Local development documents should: Flood Risk Management
Coastal and river - Use Strategic Flood Risk Assessments to guide development away from floodplains, other flooding is a areas at medium or high risk (or likely to be at future risk) from flooding, and areas where significant risk in parts development would increase the risk of flooding elsewhere of the East of - Include policies which identify and protect floodplains and land liable to tidal or coastal England. The flooding from development, based on the EA’s flood maps, supplemented by historical and priorities are to modelled flood risk data, catchment flood management plans and emerging policies in defend existing shoreline management plans and flood management strategies including managed re- properties from alignment where appropriate flooding and to locate new development - Only propose departures from the above principals in exceptional cases where suitable land where there is little or at lower risk of flooding is not available, the benefits of development outweigh the risks from no risk of flooding. flooding and appropriate mitigation measures are incorporated - Require that sustainable drainage systems are employed in all appropriate developments - Areas of functional floodplain needed for strategic flood storage in the Thames estuary should be identified and safeguarded by local authorities in their LDDs. - The East of England contains many low-lying areas at risk from flooding, the Fens make up England’s largest river floodplain, but areas of river floodplain occur throughout the region. The regions long coastline is also at risk from coastal flooding in places, particularly inland from the Wash. PPG25 refers to the more extensive areas in the region at risk of flooding. - The extensive nature of the regions vulnerability to flooding, combined with existing patterns of development, means that about 140,000 properties are within areas protected by existing flood defences. Where defences are to be maintained, new developments may be acceptable, particularly where it is making use of previously developed land. However the regions vulnerability to flooding is increasing as a result of climate change and whilst flood defences which protect settlements will be maintained and where appropriate enhanced, a more flexible approach is required in areas outside settlements which are vulnerable to tidal flooding, based on policies of managed re-alignment and relocation. Where some risk is unavoidable it must be considered at all stages of the planning process, to minimise potential damage to property and loss of life whilst avoiding harm to sites of European or international importance for wildlife. Table 2.3 – East of England Plan: Policy WAT 4 Flood Risk Management
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2.2.3 Milton Keynes and South Midlands Sub-Regional Strategy The Milton Keynes and South Midlands Sub-Regional Strategy (MKSMSRS) is a co-ordinated review of the policy for the Milton Keynes and South Midlands sub region, which is an intersection of three RSSs. The MKSMSRS took effect from September 2004 and represents revisions to the three relevant RSSs as detailed below: • East Midlands (RSS8) • East of England (RPG6 and some of RPG9, that relate to Bedfordshire, Essex and Hertfordshire) • South East (RPG9) One policy within the document relates to flood risk; this policy is set out in Strategic Policy 3: Sustainable Communities and is detailed in Table 2.4.
Strategic Policy Local development documents will: 3: Sustainable Communities
Sustainable -The growth areas will require a strategic approach and investment programme for waste water communities will be and surface water drainage management which takes a co-ordinated approach to land achieved in the Sub- drainage, nature conservation, landscape management and open space provision, so that Region through the catchment flood risk is not increased and water quality does not deteriorate as a result of the implementation of cumulative impacts of development. development in accordance with many principles but specific to flood risk is the following principle: managing and reducing demand where appropriate (e.g. demand for water) Table 2.4 - MKSMSRS: Strategic Policy 3 Sustainable Communities
2.2.4 Marston Vale Surface Waters Plan Due to the significant amount of new development in the area of the Forest of Marston Vale (situated to the south west of Bedford), the Marston Vale Surface Waters Group was set up. The group comprises of Bedford Borough Council, Bedfordshire County Council, the Environment Agency, Forest of Marston Vale, Bedford Group of Drainage Boards and the Mid Beds District Council. The group instigated the Surface Waters Plan as a strategic approach to urban flood risk management throughout Marston Vale. The Surface Waters Plan is part of 15 pilot studies being carried out by DEFRA to examine a range of different approaches to develop a more integrated urban drainage approach. The Surface Waters Plan for Marston Vale relates directly to flood risk and supports local planning policies. The main purposes of the Surface Waters Plan are as follows: • Promote the policies of the Surface Waters Group; • Support local plan policies dealing with flooding and surface water drainage; • Assist with consideration of development proposals; • Identify solutions for dealing with the impact of development pressure on watercourses and lakes; • Provide guidance to landowners and developers on approaches to management of surface water; and • Encourage schemes that result in a range of benefits including management of flood risk and enhancement of the environment.
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The Surface Waters Plan seeks to encourage landholders, developers and planners to work with the drainage authorities and the Community Forest team to devise strategic lasting solutions, generating a diverse array of benefits including increased amenity and conservation value for the management of flood risk and surface water drainage in Marston Vale.
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3. Data Collection and Review
This section will identify the information that is available for the Bedford Borough SFRA. Typical information that has been collected includes: • Sources of flood risk, and the areas likely to be affected • Flood extents - Historical extents - Hydraulic Model extents developed for Flood Risk Management studies, Standard of Protection studies, etc. • Land use and development allocation sites provide details of vulnerable areas and locations of future development. • Flood defence database - This indicates those areas that are protected from flood events. Flood risk information for the Bedford Borough is available from a number of different sources. The key contacts utilised in the completion of this Level 1 SFRA have been the Environment Agency, Anglian Water, Bedford Borough Council and the Bedford Group of Internal Drainage Boards (IDB). The information gathered during this phase has been used to assess the potential extent and frequency of flood risk, the implications of this flood risk for development opportunities and the opportunities for flood management practices which may help mitigate or reduce future flood risk. 3.1 Consultation All information obtained during the Level 1 SFRA consultation process has been detailed in Section 3.3 and will feed into the application of the Sequential Test. 3.1.1 Bedford Borough Council The planning department have been consulted in order to identify areas under pressure from development. The Bedford Borough Council’s drainage team have also been consulted in order to determine if there is any flooding (fluvial or surface water) that the other consultees were not aware of. 3.1.2 Environment Agency Bedford Borough Council’s area spans only one Environment Agency area – Central Area in the Anglian Region. This area has been consulted to obtain information on sources of flood risk, hydraulic modelling, flood defences and Flood Warning. 3.1.3 Bedford Group of Internal Drainage Boards The Bedford IDB is responsible for the majority of the low lying drains and watercourses that lie within the Bedford Borough. The IDB for this area have been consulted in order to identify areas at risk of flooding from their watercourses. Information on historic flooding, flood defences and hydraulic modelling from their watercourses has also been obtained. 3.1.4 Anglian Water The sewerage infrastructure within the Bedford Borough is maintained by Anglian Water. Anglian Water has supplied information on known surface water flooding locations, historical flooding and information on the assets that they maintain.
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3.1.5 British Waterways British Waterways have not been consulted as part of this Level 1 SFRA, as there are no British Waterway owned navigation stretches in the Bedford Borough at present. 3.2 Catchment Characteristics The Bedford Borough catchment covers 1,192 km2. There are large portions of rural land within the Bedford Borough and the heaviest urban sector is the town of Bedford. The geology of the Bedford Borough is predominantly comprised of Oxford Clay and Kellaways Beds, with minimal stretches of Cornbrash and Great Oolite.
Soils within the area vary from Efford 1, Evesham 3 and Moreton types. A small portion of the Bedford Borough is not surveyed. Loamy soils are a combination of roughly 40 % sand, 40% silt and 20% clay. Historically Bedford Borough has been subjected to a number of flood events, with the River Great Ouse providing the principal source of flooding. Defence structures have been implemented throughout the town centre to protect the built environment. Flood Warning areas are prevalent along the course of the River Great Ouse as well as in the north of Bedford Borough near Riseley. 3.3 Sources of Flood Risk Data The flood risk data used to inform this SFRA is discussed below under the relevant headings. To obtain this information, consultation with the Environment Agency, Anglian Water, Bedford Borough Council and the Bedford IDB has taken place. This flood risk data will form the basis of the methodology and approach used for the Level 2 SFRA. 3.3.1 Historic flood events Analysis of historic flood events is available for the Bedford Borough catchment for a number of watercourses. Information on historical flood events can supplement the understanding of flooding mechanisms and flood extents within a catchment. Detailed reporting of flood events is available from 1947 up until the present day. The data is both qualitative and quantitative, with the more recent events providing a greater level of depth and study. Post event reports and flood chronologies indicate that significant flood events which affected Bedford Borough occurred in 1947, 1987, 1992 and 1998. The following text outlines the flood events in the Bedford Borough catchment. 1947 There are two conflicting reports detailing the flooding that occurred in 1947. The first was taken from a report prepared for the Environment Agency as part of flow forecasting work. This report states that extensive road and property flooding occurred throughout Bedford and provides an estimated return period for the flood event in the order of 1% AEP; with a peak of around 290m3/s, whilst peak water levels in Bedford were around 26.27m AOD. However the Hydraulics Research Report EX 1966 prepared in 1989, states that the peak flow was 388m3/s with a 0.4% AEP. During a review of the hydrology in the Bedford Ouse catchment for the April 1998 flood event (Ref. 16), a review of all historical data was carried out. This review indicated that the flows predicted using the Hydraulic Research Institute model were high in comparison to other studies carried out for the 1998. It can therefore be assumed that the Hydraulics Research Institute model may be over predicting flows in Bedford during the 1947 event. Should any work be required to calibrate models to the event of 1947, a thorough review of the historic flooding during this event should be undertaken, in order to clarify the discrepancies between the two reports.
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1980 In August 1980 Bromham Brook flooded 62 homes and caravans. 1983 On the 1st May 1983 the Bromham Brook overtopped its banks and flooded seven properties; six on Brook Way, one at Wick End. A peak flow was estimated at 24m3/s based upon water marks, and a peak flow of 18m3/s was estimated using the Flood Studies Report (FSR) method. The preceding rainfall event was found to have a return period of 10% AEP. 1987 In October 1987 moderately high rainfall fell over the catchment with 20-40mm in 24 hours. This rainfall combined with a Soil Moisture Deficit (SMD) which had reached zero, flooded a number of areas; the road between Kimbolton to Stonely became impassable for 24 hours. 1992 In 1992 the Bedford Ouse catchment was subjected to intense rainfall, between 40-80mm in 24 hours, with SMD values marginally above zero. Flood waters affected the Riseley Brook with 18 properties in Riseley affected. Two industrial developments in Riseley were also flooded. In Bedford, one property was flooded. The B660 at Kimbolton had a depth of water of 250mm; on the A45 water depths reached 450mm. A public house in Turvey (a known flood risk area) flooded during this event. Properties were also flooded in Kimbolton, Great Staughton and Bedford. Sharnbrook Road was also inundated by flood waters. 1998 In Easter 1998, the River Great Ouse in Bedford overtopped its banks on Saturday 11th April. Many homes in the North of Bedfordshire and Bedford were flooded and more than 500 homes in Bedford had their electricity supply cut off as electricity supplies went down. 2003 In 2003 the IDB instigated a closure of the A1 (southbound) at Sandy due to flooding, however this is in the Mid Bedfordshire district and there was no reports of flooding within the Bedford Borough. 3.3.2 Hydraulic Modelling and Flood Outlines A number of hydraulic modelling studies have been undertaken within the Bedford Catchment. The majority of these studies have been undertaken on behalf of the Environment Agency and the IDB. Available models are summarised in the table below;
Model Name Date Originator Watercourse Bedford S105 Feb 2003 EA (Royal Haskoning) Great Ouse St Neots S105 Feb 2003 EA (Royal Haskoning) Great Ouse Kempston Prefeasibility Oct 1999 EA (Royal Haskoning) Great Ouse Study Milton Keynes Drainage Mar 2000 EA (Halcrow) Great Ouse, Ouzel Study
Clapham Prefeasibility Sep 1999 EA (Royal Haskoning) Great Ouse Study
Kimbolton Prefeasibility Aug 1999 EA (Royal Haskoning) River Kym Study
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Model Name Date Originator Watercourse Olney, Newton Aug 2002 EA (Atkins) Great Ouse Blossomville, Turvey Prefeasibility Study
Harrold, Odell, Sharnbrook Nov 2004 EA (Atkins) Great Ouse SOP
ARTS2c Bedford Ouse Feb 2006 EA (Atkins) Inter alia Great Ouse, Block River Kym, Risely Brook
St Neots Flood Defence Mar 2008 EA (Atkins) Great Ouse Scheme PAR Elstow IDB Elstow Brook Table 3.1 –Hydraulic Model Summary
The River Great Ouse reaches that have been modelled on behalf of, or by the Environment Agency include Bedford & St Neots, Clapham & Kempston, Goldington, Harrold, Odell, Sharnbrook, Kimbolton, Olney, Newton Blossomville, Turvey, and St Neots. Riseley Brook has also been subject to hydraulic modelling on behalf of the Environment Agency. The Elstow Brook has been hydraulically modelled on behalf of the IDB. The hydraulic model and the flood outlines were not available at the time of this Level 1 SFRA however this information will be available for the Level 2 SFRA. All of the models have been produced from detailed cross-sectional and structure surveys and calibrated to event data. Flood outlines are available for these models and can be viewed in Appendix A.7. 3.3.3 Anglian Water DG5 Register The DG5 register lists the known locations of flooding from sewers. For the Bedford Borough, this information has been obtained from Anglian Water. The key locations that are affected are listed below, as is the type of flooding. Appendix B.1 holds the full detailed DG5 register for the Bedford Borough. Consultation with Anglian Water revealed the following locations are at risk and have been added to the DG5 register. These are detailed in Appendix B1 for information purposes.
Town/City Street/Road Type of Sewer Bedford Bromham Road, Bushmead River, combined sewer, foul sewer, surface Avenue, Cardington Road, water, foul and surface water. Cauldwell Walk, Conduit Road, Some sources of flooding are unknown High Street, Midland Road, Park Avenue, St Augustine’s Road, St Cuthbert’s Street, St Pauls Square, Cauldwell Court, Larkway Clapham High Street, Fetlock Close Foul sewer
Kempston Beatrice Street, High Street, Surface water, foul sewer, SW sewer, Spruce Walk, The Sycamores, possible cross connection with SW Walnut Walk Oakley Church Lane Foul (or river) Table 3.2 – Anglian Water’s DG5 register: Key locations and type of sewer
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3.3.4 Flood Warning Flood Warning has been in operation in the Bedford Borough for a number of years as a means of reducing the impacts of flooding. A range of systems have been in operation in various parts of the catchment. The Environment Agency offers Flood Warnings to all properties that lie within a Flood Warning area. However only 25 - 30% of these properties have taken up the offer and receive Flood Warnings. Within the Bedford region the use of local TV, radio stations as well as the Floodline Warnings Direct (FWD) service are the primary sources of Flood Warning dissemination. The Environment Agency system aims to publicly issue Flood Warnings with a minimum 2hr lead time. There are three stages of warning for this process, these are: • Flood Watch • Flood Warning • Severe Flood Warning A Flood Watch is the lowest level of Flood Warning and warns of out of bank flooding to low lying areas and agricultural land. Individual properties are not usually warned if they are located in a Flood Watch area. Flood Watch areas are in place for the River Great Ouse, the Flood Watches cover the following areas within the Bedford Ouse Catchment: • The Rivers Kym and Til and the Alconbury, Ellington and Riseley Brooks • The River Great Ouse in Cambridgeshire and parts of East Bedfordshire • River Ouzel and Clipstone Brook in Bedfordshire and Buckinghamshire • Rivers Ivel, Flit, Hit, Purwell and Hiz in Bedfordshire A Flood Warning is issued to properties that lie within a Flood Warning Area. A Severe Flood Warning is the highest level of warning and is issued to individual properties that lie within Flood Warning areas to warn of imminent flooding and risk to properties and life. There are eight Flood Warning areas in the Bedford Catchment that are the responsibility of the Environment Agency: • The River Great Ouse at Olney; • The River Great Ouse from Turvey to Sharnbrook∗; • The River Great Ouse from Sharnbrook to Kempston∗; • River Great Ouse from Bedford to Offord; • The River Til, the Riseley Brook and the Pertenhall Brook; • The River Ouzel in the Leighton Buzzard area, down to Newport Pagnell in Buckinghamshire; • The Clipstone Brook; and • The River Ivel and River Flit in the Shefford area down to and including Biggleswade. More specific Flood Warnings are in place to close roads; these are known as operational warnings. The Environment Agency advise the Local Authority when a road is expected to be flooded, it is then the decision of the Local Authority to close the road. These operational warnings are issued at the following locations: • Harrold - River Great Ouse
∗ Denotes Flood Warning Area currently under review by the Environment Agency
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• Felmersham - River Great Ouse • Radwell - River Great Ouse • Oakley - River Great Ouse There is also a navigation warning in place which gives strong stream advice for the navigational stretch of river between Bedford to Earith. 3.3.5 Flood Zone Definition Environment Agency Flood Map The Environment Agency’s Flood Map was first published on the internet in October 2004. The flood maps show the best estimate of flood extents for the undefended 1% AEP and 0.1% AEP fluvial floodplain and 0.5% AEP and 0.1% AEP tidal floodplain. The Flood Map outlines have been derived using a combination of a generalised model derived as part of the Flood Zone Project (a high level national mapping programme), more detailed hydraulic modelling and historical flooding outlines. The Flood Map outlines therefore have a varying degree of accuracy dependent on the quality of the inputs and in particular the availability of detailed hydraulic modelling. The Flood Map is updated on a quarterly basis as the Environment Agency’s knowledge of flooding is improved through detailed modelling studies, recent flood events and data from river level and flow monitoring stations. The Flood Map presents flood risk in accordance with the PPS25 Flood Zones 1, 2 and 3. Appendix A.3 displays the existing Environment Agency Flood Maps for Zones 2 and 3 and Appendix A.8 shows more detailed Flood Zone Mapping. Previous flood mapping projects provide flood extents for a number of watercourses within the Bedford Borough region. The Environment Agency’s Flood Mapping is also available for preliminary mapping purposes. In some instances it is possible to outline the flood extents of historical flood events for reference purposes. It should be noted that although hydraulic modelling of the Elstow and Harrowden Brooks has been carried out, the resultant flood outlines are not currently incorporated into the Environment Agency Flood Maps for Zones 2 and 3 as this work is currently ongoing. PPS25 Flood Zones PPS25 splits the Environment Agency’s Flood Map into four separate Flood Zones. These zones should be used when determining the appropriateness of proposed development uses when considering flood risk through the application of the Sequential Test. These Flood Zones represent flooding without flood defences in place. Table D1: Flood Zones and Table D2: Flood Risk Vulnerability Classification, within Annex D of PPS25 respectively defines these Flood Zones and describes the appropriate land use vulnerabilities for each zone. The Land Use Vulnerability table has been reproduced from table D2 of PPS25 and is shown in Table 3.3. A summary of each Flood Zone and land use is also provided in Table 3.3 and Table 3.4.
Land Use Vulnerability Type of Development
Essential Infrastructure • Essential transport infrastructure (including mass evacuation routes) which has to cross the area at risk, and strategic utility infrastructure, including electricity generating power stations and grid and substations
Highly Vulnerable • Police Stations, Ambulance Stations, and Fire stations, Command Centres and telecommunications installations required to be operational during flooding. • Emergency dispersal points • Basement dwellings • Caravans, mobile homes and park homes intended for permanent residential use • Installations requiring hazardous substances consent
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Land Use Vulnerability Type of Development
More Vulnerable • Hospitals • Residential Institutions such as care homes, children’s homes, social services homes, prisons and hostels • Buildings used for: dwelling houses; student halls of residence; drinking establishments; nightclubs; and hotels • Non-residential uses for health services, nurseries and educational establishments • Landfill and sites used for waste management facilities for hazardous waste • Sites used for holiday or short let caravans and camping, subject to specific warning and evacuation plans.
Less Vulnerable • Buildings used for: shops, financial, professional, and other services; restaurants and cafes, hot food takeaways; offices; general industry; storage and distribution; non-residential institutions not included in ‘more vulnerable’; and assembly and leisure • Land and buildings used for agriculture and forestry • Waste treatment (except for landfill and hazardous waste facilities) • Minerals working and processing (except for sand and gravel working) • Water treatment plants • Sewage treatment plants (if adequate pollution control measures in place)
Water Compatible Development • Flood control infrastructure • Water transmission infrastructure and pumping stations • Sewage transmission infrastructure and pumping stations • Sand and Gravel workings • Docks, Marinas and Wharves • Navigation facilities • MOD defence installations • Ship building, repairing and dismantling, dockside fish processing and refrigeration and compatible activities requiring a waterside location • Lifeguard and coastguard operations • Amenity open space, nature conservation and biodiversity, outdoor sports and recreation and essential facilities such as changing rooms • Essential ancillary sleeping or residential accommodation for staff required by uses in this category, subject to specific warning and evacuation plans Table 3.3 - Flood Risk Vulnerability Classification (based on Table D2 of PPS25) Flood Zone 1 is defined as having a ‘Low Probability’ of flooding and incorporates areas where the annual probability of flooding is lower than 0.1%. PPS25 imposes no constraints upon the type of development within Flood Zone 1. Flood Zone 2 is defined as ‘Medium Probability’ with an annual probability of flooding between 0.1 and 1.0% for fluvial and 0.1 and 0.5% for tidal and coastal flooding. PPS25 recommends that Flood Zone 2 is suitable for most types of development (see Table 3.4Table 3.3) with the exception of ‘Highly Vulnerable’ land uses as defined in table D2 of PPS25. Flood Zone 3 is defined as ‘High Probability’ with an annual probability of flooding of 1.0% or greater for fluvial and 0.5% and greater for tidal or coastal. PPS25 recommends that appropriate development is based upon a further classification of Flood Zone 3 into 3a High Probability and 3b Functional Floodplain (where water has to flow or be stored in times of flood). Table 3.3 and Table 3.4 details the development types that PPS25 allows within Flood Zone 3a and Flood Zone 3b. Greater constraints are placed upon development within Flood Zone 3; refer to Table 3.4 for details.
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Flood Risk Essential Water Highly More Less Vulnerability Infrastructure Compatible Vulnerable Vulnerable Vulnerable Classification (see Table D2 of PPS25)
Zone1 9 9 9 9 9 Low Probability
Zone2 Exception Test 9 9 9 9 Medium required Probability
Zone 3a Exception Test Exception Test 9 x 9 High required required Probability
Zone 3b Exception Test ‘Functional 9 x x x required Floodplain’ Flood Zone (see Table D1 of PPS25)
9 Development is appropriate
X Development should not be permitted Table 3.4 – Appropriate Development for each Flood Zone (based on Table D3 of PPS25)
3.3.6 Bedford Group of Drainage Boards (IDB) Watercourse Surveys The watercourse surveys provide cross-section information and can be incorporated into hydraulic modelling. Maps are provided which detail the location of each of the cross-sections along with a photograph at the specific site. Cross-section schematics are available at these points. These watercourse surveys may be beneficial for the Level 2 SFRA, if further models are required. 3.3.7 Flood Event Anecdotal Evidence The IDB have provided known locations of flooding from their records. Historic flooding locations include: • Bromham Weir • Kempston Weir • Bedford Telytone Duckmill Weir • Cardington Sluice • Castle Mill Sluice • Willington Weir • Barford Weir • Roxton Weir • Lovell Road. The culvert system from Jubilee Park causes road flooding and is assumed to be blocked. • Rays Close. Flooding from King's Ditch affects the road and an elderly care home has been evacuated previously. • Dame Alice Harpur School. The Environment Agency controls a penstock at this location to isolate flows in Kings Ditch from high flood levels on the main Great Ouse. • Sharnbrook. The IDB is aware that Mill Road underneath the railway is prone to flooding (closing the road) possibly from blockages in the highway drains or at the outfall with the river.
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• Wilden. The road floods due to surface water running from the north. • Wilden. General road flooding has been observed. • Wilden High Street/East End Lane. Flooding occurs from the watercourse and land to the north. • Colesden. A pinch point has been noted at the road culvert. Out of bank flooding occurs at this location. • Wilstead. The IDB is aware that the village lies very wet and is prone to flooding. • Cotton End. The IDB is aware that the village lies very wet and is prone to flooding. • Cople. This village has been identified by the IDB as having flood risk problems. 3.3.8 Structure Inspection Reports The Environment Agency has provided structure inspection and diving reports for the following resources with accompanying drawings and photographs of the structures: Castle Mills Sluice, Harrold Middle Weir, Harrold Mill Weir, and Harrold Top Weir. This information will aid the appraisal of the current flood defence structure required for the Level 2 SFRA. 3.3.9 Existing Flood Defences The IDB maintain a number of flood defence structures within the Bedford Borough. The full list is given in Appendix B.2. The list comprises flood storage areas, embankments, weirs, outfalls and sluices. The description of the structure is given along with the other information such as the owner/maintainer, residual life and condition. The IDB also identifies that the watercourse channels and their conveyance capacity area are an important part of the flood risk management system. These are categorised as 1, 2, and 3, where maintenance works such as weed cutting and dredging are carried out yearly, every two to five years, and greater than five years, respectively. The Environment Agency maintains a variety of flood defence structures throughout the Bedford Borough. These are recorded within the National Flood and Coastal Defence Database (NFCDD). The structures include channels that are maintained in order to protect against fluvial flooding (e.g. revetments, raised walls etc) and man-made raised defences. Thorough descriptions of the structures are given including materials, dimensions, design levels and conditions. Asset photographs are available for a selection of structures. This Level 1 SFRA assumes that all areas at risk of flooding are undefended. The Level 2 SFRA will detail the defended and undefended areas for all allocated developments that lie in areas at risk of flooding. These areas will be subject to further detailed assessment and will assist in the appraisal of flood defences required for the Level 2 SFRA. 3.3.10 Development Plans, Policy and Guidance The following Development Plans, Policy and Guidance documents have been provided to aid with both the Level 1 and the Level 2 SFRA: • The Marston Vale Surface Waters Plan (Ref. 13) • The Draft East of England Plan (Ref. 15) • Core Strategy & Rural Issues Plan (Ref. 14) • Bedford Town Centre Area Action Plan (Ref. 3) • Great Ouse Catchment Flood Management Plan (Ref. 11) • Bedford Borough Local Plan 2002 (Ref. 4) 3.3.11 Existing Studies on Flood Risk The Bedford Town Centre SFRA (Ref. 3) was completed in 2006 by WSP. The document examines the risk of flooding from: key watercourses, structures, flood defences, modelled flood
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levels, flooding mechanisms, historic flood levels, sewers, groundwater, drains, overland flows, and climate change. The information in this SFRA will complement the Bedford Town Centre SFRA. In 1993 HR Wallingford carried out a flood modelling study of a reach of the River Great Ouse from Kempston to Willington (Ref. 6). A range of design events were examined (see Table 4.2 of the report) and the extent of flooding commented upon (see Table 4.3 of the report). The Environment Agency has recently developed hydrological and hydraulic models for the Bedford Ouse catchment. These models are currently being used to provide flood forecasts for the Bedford Ouse catchment. As part of the development of the Bedford Ouse forecasting model a series of reports were produced. The reports produced are as follows: • Data review – review of hydrometric, topographic and flood history data sets for the Bedford Ouse (Ref. 7); • Catchment conceptualisation (Ref. 8); • Model solution – description of the generic modelling solutions that will be used to develop a catchment model of the Bedford Ouse (Ref. 9); • Model set-up – description of how the modelling solutions described in previous reports will be applied to Bedford Ouse. The key focus is a schematic of the model as it was built (Ref. 10). 3.4 Data Summary Flood Risk Data for the Bedford Borough is available from a number of sources, as listed previously. All of the data that has been collected for this Level 1 SFRA has been assessed in the sections above. This section details the information that has been used to create the Level 1 SFRA mapping for the area which will aid the application of the assessment of flood risk and the Sequential Test which will be carried out in the Level 2 SFRA. 3.4.1 Data Analysis Data analysis takes the form of Flood Zone Mapping of various return period events, coupled with historical flood extents. The impact of climate change on these events has been assessed and accounted for in a series of figures. Potential sustainable mitigation options are discussed, making reference to the geology and topography of the area along with specific flood risk locations. 3.4.2 Data Mapping Flood locations and flood extent mapping is available from a number of sources as listed below: • flood reports which cover the Bedford Borough Catchment; • hydraulic modelling studies; • historic flood events; • the Environment Agency; • the IDB and • Anglian Water data The information that has been obtained has been mapped and could be used by Bedford Borough to aid the application of the Sequential Test. 3.4.3 Flood Zone Mapping The Flood Zone Mapping carried out for this Level 1 SFRA has been completed from all data sources. The Flood Zone Mapping will form the basis of the Sequential Test and allows for designated development areas to be assessed in terms of flood risk both in the present as well as
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in the future. In order to identify further areas for development in terms of flood risk, the whole of the Bedford Borough is shown in detail on the maps in Appendix A. The data used in the mapping is as described in section 3.3. The following appendices should be consulted for the mapping element of the study: • Appendix A.1 holds the location map for the study • Appendix A.2 details the locations of the development allocations within the Adopted Bedford Borough Local Plan. The map does not include those development allocations that lie within the Bedford TCAAP and therefore already within the TCAAP SFRA. • Appendix A.3 shows the locations of the Flood Zones 2 and 3 • Appendix A.4 holds the information for the structures overview. • Appendix A.5 holds the information for the Flood Warning overview. • Appendix A6 shows the Bedford IDB catchment, the watercourses they maintain and the assets that they maintain. • Appendix A.7 shows the mapping index for the detailed mapping that has been carried out. • Appendix A.8 provides detailed mapping for the flood risk of the Bedford Borough, including Flood Zones, modelled flood outlines, historic flood extents and spot locations for flooding from IDB and Anglian Water • Appendix A.9 provides detailed mapping to show the assets and structures within the Bedford Borough. The detailed mapping also includes details of structures and assets maintained by the Environment Agency, IDB, and Bedford Borough.
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4. Flood Risk
Bedford Borough Council have already allocated sites within the Borough that have been subject to site specific FRAs and have been committed to development, these areas are shown in Appendix A.2. For further information on these areas, the site specific FRAs should be consulted. This section will summarise the flood risk that has been identified within Bedford Borough as a result of the data collected for this Level 1 SFRA. It will also summarise the information obtained on flood defences and provide some information on the sensitivity to flood risk within the Bedford Borough. 4.1 Sources of Flooding
4.1.1 Fluvial Overview The primary source of flood risk in Bedford Borough is fluvial flooding. The River Great Ouse passes through the centre of Bedford and through many of the surrounding villages. In addition to the River Great Ouse there is a risk from the River Til and the Riseley and Pertenhall Brooks to the north of Bedford. The Elstow Brook to the south of Bedford poses a flood risk. There are also numerous smaller watercourses and ditches that cross the Borough. Appendix A.3 shows the location and flood risk (from Environment Agency Flood Zones) of these watercourses. There is some risk of flooding in parts of the following built up areas • Bedford • Pertenhall • Kempston • Felmersham • Bromham • Oakley • Clapham • Great Staughton • Sharnbrook • Wilden • Harrold • Riseley
• Great Barford • Radwell
Delineation of Flood Zone 1 – Low Probability Flood Zone 1 is classed as low probability flood risk and is defined as those areas of land which is outside of the shaded Flood Zone 2 and Flood Zone 3 flood risk areas (as defined below). Delineation of Flood Zone 2 – Medium Probability Flood Zone 2 is classed as medium probability flood risk and is defined as those areas of the Borough which are situated between the 0.1% AEP and the 1% AEP fluvial flood extents. The Flood Zone 2 medium probability is defined in accordance with the Environment Agency Flood Zone Map, however, there are areas within this Flood Zone that have been updated from detailed hydraulic modelling and there are further areas that would benefit from the Flood Zone 2 map being updated with the further detailed hydraulic modelling work that has been carried out. Delineation of the Flood Zone 3a – High Probability and Flood Zone 3b – Functional Floodplain Flood Zone 3a is classed as high probability flood risk and has been defined as those areas that are situated below (or within) the 1% AEP fluvial flood extent.
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Flood Zone 3b is classed as functional floodplain and is defined as ‘land where water has to flow, or be stored in times of flood’. PPS25 defines this as ‘land which would flood with an annual probability of 5% or greater in any year and is designed to flood in an extreme (0.1%) flood, or at another probability to be agreed between the LPA and the Environment Agency, including any water conveyance routes’. For the purposes of this Level 1 SFRA, it was agreed at a meeting held on the 11th January 2008 (Ref. 5) that any of the Bedford Borough area lying within Flood Zone 3 would be classed as ‘Functional Floodplain’ and therefore lying within Flood Zone 3b. The Level 2 SFRA will require Flood Zone 3 to be refined to Flood Zone 3a and 3b by utilising and improving hydraulic models. This can also be achieved by incorporating the information collected during this Level 1 SFRA into the Flood Zone maps supplied by the Environment Agency. 4.1.2 Sewers Surface water flooding has the potential to contribute a significant flood risk in urban areas due to the rapid runoff rates associated with urban land use and the volume of water that flows into the sewer systems in a relatively short space of time. Flooding of sewers can occur when the artificial drainage is overwhelmed hydraulically, becomes blocked, or suffers structural failure or pump failure. Blockage and structural failure incidents tend to be isolated and unpredictable and therefore inappropriate for a strategic level analysis. A review of areas where the sewer system has been overwhelmed can potentially identify limited capacity of the drainage system or where the system does not provide an adequate level of service. Anglian Water have provided their DG5 register which details locations of surface water and foul water flooding (see section 3.2.3). Appendix A.8 shows detailed maps of known surface water and foul water flooding locations within the Bedford Borough. The areas of Clapham, Kempston, Bedford and Oakley have previously experienced foul and surface water flooding from the Anglian Water system. 4.1.3 Groundwater Flooding can occur as a result of high groundwater levels if the water table rises above the ground level. The majority of the underlying geology in the area is Oxford Clay, a non aquifer, however, the Oxford Clay has been eroded in some area of the River Great Ouse valley exposing the underlying Great Oolite deposits, which have significant groundwater resources. The exposure of these deposits could cause groundwater flooding. The Environment Agency, Bedford Borough Council and the IDB have no records of historical groundwater flooding incidents in the Bedford Borough. The Great Ouse CFMP does not identify any groundwater flooding within the Bedford Borough area. Ref: 12 shows that the village of Keysoe was affected by groundwater flooding in 2000-2001, however no other information has been supplied to verify this. At a meeting held on 11th January 2008 (Ref. 5), the Environment Agency raised potential groundwater issues at Biddenham Loop, where there may be deposits of Great Oolite exposed. This area has already been allocated for development and therefore groundwater flood risk will have been addressed. The site specific FRA is referred to for further information. It has been highlighted by the IDB that the villages of Wilstead, Cotton End and Cardington lie very wet and it is considered that this is possibly due to high water tables. The IDB have some concerns in the Kempston area, where there have been recent problems of waterlogged gardens. It is recommended that the Level 2 SFRA should look at the effects of groundwater movement in the area as the change in land use may affect the flow of groundwater which could potentially give
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rise to flooding. The Level 2 SFRA should also investigate further the groundwater issues in the villages identified by the IDB to determine if these areas are subject to groundwater flooding. 4.1.4 Canal Infrastructure There are no canals that flow through Bedford Borough and therefore flood risk from canal infrastructure is not applicable at this stage. There is a stretch of navigational river along the River Great Ouse between Bedford and Offord; this is managed and maintained by the Environment Agency and does not pose a flood risk other than the fluvial flooding already considered. There are proposals to construct a section of canal from Milton Keynes to Bedford which would link the Grand Union Canal to the River Great Ouse. It is proposed that the new canal link is investigated further during the Level 2 SFRA, as part of a detailed assessment of flood hazard. 4.1.5 Reservoirs Flooding from reservoirs can occur when water retaining structures fail. All large reservoirs are covered by the Reservoirs Act and are subject to regular safety inspections. A very low residual risk from these reservoirs remains if they were to fail unexpectedly, however this is considered a risk that is managed by the water companies or the Local Authority. There are several balancing lakes and whilst the list below is not an exhaustive list these are located as follows: • Thurne Way – Bedford • Douglas Road – Bedford • Chantry Road – Kempston • Woburn Road – Bedford • Stewartby Lake • Marsh Leys • The Wixams • Great Barford • A6 Park and Ride • Interchange Retail Park There is also a reservoir/storage area owned by Anglian Water located at Mowsbury/Cleat Hill. There are also a number of proposed reservoirs/balancing ponds which would be located at proposed development sites in Cardington. From the data collected during this Level 1 SFRA, there is no evidence of flooding from any of the above balancing lakes. It has been assumed that there is no flood risk from these balancing lakes as long as they are maintained to a good standard. Anglian Water has confirmed that they will be adopting a closed balancing facility at Acacia Road in Bedford. Due to the complexity of locating all balancing facilities within the Borough and the low flood risk associated with them at a strategic level, the risk of flooding from existing and proposed individual ponds/systems should be addressed within site specific FRAs.
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4.2 Flood Defences Both the Environment Agency and the IDB own and maintain defences within Bedford Borough. A map showing the location of flood defences is provided in Appendix A.8. This map shows that there are no formal flood defences on the River Great Ouse, with the exception of the main Bedford Town Centre (the Bedford TCAAP should be referred to for more information on these defences). Both the IDB and the Environment Agency have supplied a comprehensive list of flood defences and assets within the Borough. The lists provide details such as; description, asset condition and residual life of the defence. This list will aid the Level 2 SFRA, whereby it is recommended that the information on the lists relating to condition and standard be assessed in further detail. Future policy for the maintenance and standard of these defences, with an allowance for climate change, will also be recommended as part of the Level 2 SFRA. In addition to formal flood defences, the Environment Agency operates a Flood Warning Service to some of the properties that lie within Flood Zone 3 (see Appendix A.5). The Flood Warning Service has operated within the Borough for many years and will continue to do so under the Environment Agency’s remit to offer a Flood Warning Service to areas as shown within Appendix A.5. The Flood Warning Service is continually reviewed by the Environment Agency following fluvial flood events. The Environment Agency are also in the process of reviewing and updating their Flood Warning Flood Risk Areas within the Bedford Borough area. 4.3 Flood Risk Sensitivity The following section details the potential risk of flooding due to climate change, increase in urban land use and changes in land management practices. 4.3.1 Climate Change PPS25 and other Planning Policies, such as PPS1 ‘Delivering Sustainable Development’, clearly recognise the need for future growth to consider the impacts of climate change to ensure development is undertaken in a sustainable manner. Future climate change in England will impact upon both sea levels (causing sea levels to rise), and rainfall intensities (which will intensify). Detailed figures on climate change allowances are presented in Tables B.1 and B.2 of PPS25 respectively. Within the Bedford Ouse sub-catchment existing properties currently at risk of flooding would therefore be at an increased risk due to climate change and further properties in areas that are not currently at risk may be prone to flooding in the future. A 1% AEP (Flood Zone 3) flood outline including an allowance for climate change was not available for the Bedford Borough area. Therefore for the purposes of this assessment, the 0.1% AEP flood outline (Flood Zone 2) has been used as a surrogate for the 1% AEP plus climate change outline (Ref. 5). Climate change flood outlines will be determined during the Level 2 SFRA, in line with the recommendations of PPS25. 4.3.2 Increased Urban Development The impact of increased urbanisation, if uncontrolled, is likely to create an increase in flood risk. Storm runoff from impervious surfaces, routed rapidly by artificial drainage networks can increase flood peaks in watercourses downstream of new urbanised areas. There is also an increased risk of localised ‘flash flooding’ during intense rainstorms. Urban growth will, therefore, increase surface water runoff rates and volumes if not properly controlled. However, new developments are unlikely to increase surface water flood risk for the following reasons: • Some new development is likely to be located on ‘brownfield’ sites. These sites will already have connections to the drainage network. Therefore, unless the land use significantly increases the impermeable surface area, new development is unlikely to increase surface
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water flood risk. There is an exception to this, whereby low density residential areas of large houses, with extensive gardens, are replaced with high density developments of flats or smaller houses. With the use of Sustainable Drainage Systems (SUDs) these potential adverse effects can be avoided. • For any ‘greenfield’ allocations, surface water discharges from any new developments into watercourses would be controlled by the planning authority with technical leadership and guidance from the Environment Agency and the IDB to ensure that the existing greenfield runoff rates are maintained post development. In addition to this, developers would have to approach Anglian Water to determine if they have the capacity to allow new development to connect to the drainage system. Anglian Water would either allow the connection if capacity allows, or they would inform the developer that an increase in capacity is required. The developer would then be required to cover the costs of increasing the capacity prior to development taking place and allowing connection to the Anglian Water system. • Any additional development carried out in the ‘Functional Floodplain’ will lead to an increase in flood risk and require further mitigation. As part of the Level 1 SFRA, it has been assumed that the ‘Functional Floodplain’ is the same as the Flood Zone 3 outline Section 4.3.3 below identifies surface water mitigation options in the event that surface water flood risk is increased. 4.3.3 Potential Sustainable Mitigation The form and function of SUDS to be used within a development, is heavily dependent on catchment characteristics. The topography and geology of the area will determine which form of SUDS would be most beneficial for the site; whether to assist the movement of water through infiltration, or to store excess flows. Table 4.1 details the types of SUDS options that could be developed within Bedford Borough.
SUDS Component Utilisation on Development Site Rainwater Rainwater from roofs is collected and used for irrigation purposes. This harvesting may be undertaken with the use of water butts or underground storage facilities. Infiltration Porous paving can be used for the majority of hardstanding areas. Where possible this should incorporate full infiltration. The use of soakaways and deep infiltration systems is not suitable for areas where shallow groundwater levels exist (<2m from ground level). Subsurface storage This could be utilised alongside outflow controls to attenuate surface water during heavy storms, reducing peak flow rates significantly. Retention If the size of the development site permits this method can be utilised, Ponds/Detention however, there are health and safety risks regarding open water. Basins Wetlands This would be dependent upon the size of the development. It is acknowledged that many of the developments will not be able to use this SUDS method Green Roof Dependent upon the type of development green roofs may be feasible and should be considered. Swales Green space on a development site could be limited and the land take required for ‘optimum’ swale geometry may prevent their use on many developments. The location and topography may also prevent the use of Swales. Table 4.1 – SUDS Options that may be utilised within Bedford Borough Opportunities for SUDS that use infiltration may be limited in areas where; there is a low permeability of soil; the water table is shallow; the groundwater under the site may be at risk; there is a potential for land contamination; and the infiltration of water in to the ground may adversely affect ground stability (Ref. 17). A majority of the Borough has an underlying geology of Oxford Clays and only some areas are exposed by the more permeable Great Oolite. It is recommended that any site specific developments that may utilise SUDS infiltration techniques should be subject to thorough geological investigations to determine if infiltration techniques are suitable for the site.
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As part of ongoing development within Bedford and the surrounding area, a strategic surface waters plan was published on behalf of the Marston Vale Surface Waters Group which follows PPG25 guidance. In the light of the new PPS25 guidance this plan is to be reviewed. The plan seeks to promote a series of policies to encourage a sustainable and integrated approach to the major development that is proposed for the Marston Vale area. The Marston Vale Surface Waters Plan seeks to mitigate surface water in the following ways: • By carrying out strategic watercourse improvements and/or providing balancing ponds to serve large areas (i.e. source control at strategic development size scale rather than individual property source control). • The strategic SUDS should be adopted by public bodies, such as the Forest of Marston Vale becoming the landowner responsible for amenity and land management and the IDB adopting the asset and using its powers to manage and maintain the flood risk management elements. Water storage and attenuation appears to be the primary focus for sustainable drainage in Bedford Borough. The IDB have highlighted that there is a series of brick pits within the south of the Bedford Borough and it is the aspiration to use these brick pits for flood storage and recharge. These brick pits would then be linked using the Elstow Brook to provide additional amenity values. Bedford Borough Council wishes to try and adopt a strategic approach to surface water mitigation. Similar to that of the Marston Vale Surface Waters Plan, it is hoped that the policy and guidance for the use of SUDS within the region will be adopted for the larger of the proposed developments. The primary function of SUDS is for managing flood risk to protect people and property. Developers must ensure that the design of any surface water system considers future maintenance and operation in perpetuity of the development. For example experience has shown that protected species can adopt SUDS facilities (such as balancing ponds) as habitat which compromises the performance of the system as maintenance can be restricted. Therefore careful consideration should be given at an early stage to the design and type of SUDS in context with other factors. The Level 2 SFRA will set out a comprehensive guide to the available options for SUDS within the Borough. 4.3.4 Land Management The volumes and rates of runoff from land into watercourses can be affected by agricultural practices, such as the removal of hedgerows and woodland areas, reshaping the landform and the provision of positive land drainage. Such practices can result in an increase in flood risk from these watercourses to areas downstream. The Great Ouse CFMP outlines a number of potential scenarios that the catchment may experience. Agricultural decline and intensification of practices, dependent upon a number of pressures and drivers of economic change, could occur within the area. In addition it is also possible that changes to farming subsidies and the increased pressure for more sustainable farming practices will lead to areas of agricultural land becoming available for other purposes, including flood storage and conservation. These scenarios would alter the land drainage character of the catchment. The meeting that took place on 11th January 2008 (Ref. 5) highlighted the fact that the Bedford Borough area would be subject to an increased area of woodland as the Forest of Marston Vale seeks to increase its woodland cover to 30% of the identified forest area by 2030. There are also policies in place to ensure that the Bedford Borough meets its target level of woodland cover, as detailed below: • East of England Plan RRS14 - Policies ENV1 and ENV4 • Bedford Borough Council - Local Plan Policies S5, NE21, NE22 & NE23
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• Core Strategy & Rural Issues Plan policies CP22 and CP24
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5. Developer Guidance
This section provides some generic guidance for developers who wish to develop within Bedford Borough. It also details some specific guidance for developers in the Key Service Centres, which have been identified by Bedford Borough Council for development. 5.1 Generic Developer Guidance The developer should consult various documents when considering developing in an area. These documents are outlined in the following sections. If the area lies within Flood Zone 2 or 3 or is in excess of one hectare, a Flood Risk Assessment is required. This section aims to guide the developer through the use of the SFRA and also explains the requirements of a Flood Risk Assessment and the level of detail required in the FRA. It also provides guidance for the types of development within each of the Flood Zones, as well as additional guidance for developing in undefended and defended floodplains and information on how to raise floor levels and provide compensatory storage if required. Guidance on the Surface Water Drainage Assessment and Appropriate Mitigation Measures is also detailed in this section. 5.1.1 How to Use the Strategic Flood Risk Assessment The Strategic Flood Risk Assessment is the assessment and categorisation of flood risk on a district wide basis in accordance with PPS25. SFRAs refine information on the probability of flooding, taking other sources of flooding and the impacts of climate change into account. The SFRA provides the basis for applying the Sequential Test and the Exception Test where consideration needs to be given to the impact of the flood risk management infrastructure on the frequency, impact, speed of onset, depth and velocity of flooding within the Flood Zones considering a range of flood risk management maintenance scenarios. A developer should consider flood risk issues at a site as early as possible. The SFRA can be used to provide an indication of the likely flood risk issues at a site from all sources of flooding. Developers should identify whether the development site has been allocated for that type of land use in the Local Development Documents or the Local Plan. 5.1.2 When is a Flood Risk Assessment Required? A Flood Risk Assessment (FRA) will be required to accompany planning applications for:
• any development proposals of 1 hectare or greater in Flood Zone 1 • any development proposals in Medium Probability Flood Zone 2 • any development proposals in High Probability Flood Zone 3 The FRA should identify and assess the risks of all sources of flooding to and from the development, taking into account climate change and demonstrate how the risk will be managed. A FRA will also be required where the proposed development or change of use to a more vulnerable class may be subject to other sources of flooding or where the Environment Agency, Internal Drainage Board and/or other bodies have indicated that there may be drainage problems. 5.1.3 Standard Flood Risk Management Guidance for Developers The broad aim of the Planning Policy Statement 25 is to reduce the number of people and properties within the natural and built environment at risk of flooding. To achieve this aim, planning authorities are required to ensure that flood risk is properly assessed during the initial planning stages of any development. Responsibility for this assessment lies with developers and they must demonstrate the following: • Whether the proposed development is likely to be affected by current or future flooding from any source.
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• Whether the proposed development will increase flood risk elsewhere. • Whether the measures proposed to deal with any flood risk are sustainable. The developer must prove to the Local Planning Authority and the Environment Agency/IDB that the existing flood risk or flood risk associated with the proposed development can be satisfactorily managed. The detail to be provided by a FRA will depend on where the proposed site fits within the development framework, particularly on its justification against the sequential test, described in the SFRA. Development should follow the standard Flood Risk Assessment approach provided by the Environment Agency and CIRIA, as follows: • National Standing Advice to Local Planning Authorities for Planning Applications - Development and Flood Risk in England’ (June 2004) • CIRIA Report C624 “Development and Flood Risk – Guidance for the Construction Industry” (2004). The general requirements of a FRA are listed in Appendix E of PPS25 and within the Practice Guide to PPS25. Further guidance on the level of detail required for a FRA can be found in the Environment Agency’s Flood Risk Assessment guidance notes available at http://www.pipernetworking.com/floodrisk/index.html 5.1.4 Guidance for Development within Each Flood Zone A FRA should be commensurate with the risk of flooding to the proposed development. For example, where the risk of flooding of the site is negligible (Flood Zone 1 Low Probability) there is little benefit to be gained in assessing the potential risk to life and/or property as a result of flooding. The particular requirements for FRAs within each of the Flood Zones delineated within PPS25 are outlined below. Flood Zone 1 Low Probability There are generally no fluvial or coastal flood risk related constraints placed upon future development within Zone 1 Low Probability according to PPS25; however it is important to recognise that if development is not carefully managed within this zone it may adversely affect the existing flooding regime. The risks of alternative sources of flooding (e.g. groundwater, pluvial) need to be considered. The proposed development should also consider surface water runoff to ensure that there are no detrimental effects to existing development and where possible the runoff is reduced through sustainable drainage systems. Flood Zone 2 Medium Probability To satisfy the requirements of the Sequential Test, PPS25 recommends that development within Flood Zone 2 should be restricted to ‘essential infrastructure’, ‘water compatible’, ‘more vulnerable’ or ‘less vulnerable’ land uses (see Table 3.3 for a list of types of development appropriate for these land use classifications). Where non-flood risk related planning matters dictate that highly vulnerable development should be considered further within Flood Zone 2 it will be necessary to carry out the Exception Test. PPS25 states that for the Exception Test to be passed: 1. it must be demonstrated that the development provides wider sustainability benefits to the community that outweigh flood risk, informed by a SFRA where one has been prepared. 2. the development should be on developable, previously-developed land or, if it is not on previously developed land, that there are no reasonable alternative sites on developable previously-developed land; and 3. a FRA must demonstrate that the development will be safe, without increasing flood risk elsewhere, and, where possible, will reduce flood risk overall.
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The risks of alternative sources of flooding (e.g. groundwater, pluvial) need to be considered. The proposed development should consider surface water runoff to ensure that there are no detrimental effects to existing development and where possible the runoff is reduced through sustainable drainage systems. As part of the FRA, it will be necessary to demonstrate that the residual risk of flooding can be effectively managed and a planned evacuation route or safe haven can be provided.
Flood Zone 3a High Probability To satisfy the requirements of the Sequential Test, PPS25 recommends that development within Flood Zone 3a should be restricted to ‘Less Vulnerable’ and ‘Water Compatible’ land uses (see Table 3.3 for a list of types of development appropriate for these land use classifications). Where non-flood risk related planning matters dictate that ‘More Vulnerable’ development or ‘Essential Infrastructure’ should be considered further within Flood Zone 3a, it will be necessary to carry out the Exception Test (as discussed above). An FRA should include the following: • The vulnerability of the development to fluvial and/or tidal flooding as well as to flooding from other sources. • The impact of climate change over the lifetime of the development on the flooding regime, i.e. maximum water levels, flood extents and flow paths. • The effect of the new development on surface water runoff ensuring that there are no detrimental effects to existing development and where possible that runoff is reduced through the use of appropriate sustainable drainage systems. • Demonstration that residual risks of flooding, after existing and proposed flood management and mitigation measures are taken into account, are acceptable. • Demonstration that dry access can be provided to enable the safe evacuation in the event of flooding or where this is not achievable a safe haven can be provided.
Flood Zone 3b Functional Floodplain PPS25 recommends that development within Flood Zone 3b should be restricted to ‘water compatible’ land uses (see Table 3.3 for a list of types of development appropriate for these land use classifications). Where non-flood risk related planning matters dictate that ‘Essential Infrastructure’ should be considered further within Flood Zone 3b it will be necessary to carry out the Exception Test (see above for details). A FRA should include the following: • The vulnerability of the development to fluvial and/or tidal flooding as well as other sources, e.g. groundwater, sewer, surface water, critical infrastructure failure. • The impact of climate change over the lifetime of the development on the flooding regime, i.e. maximum water levels, flood extents and flow paths. • The effect of the new development on surface water runoff ensuring that there are no detrimental effects to existing development and where possible that runoff is reduced through sustainable drainage systems. • Demonstration that residual risks of flooding, after existing and proposed flood management and mitigation measures are taken into account, are acceptable. • Demonstration that dry access can be provided to enable the safe evacuation in the event of flooding or where this is not achievable a safe haven can be provided.
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5.1.5 Additional Guidance This section provides additional information for developers wishing to develop in areas of undefended and defended floodplain, as well as how to raise floor levels and provide compensatory storage where required in areas such as these. Undefended Floodplain Areas at risk of flooding need to be assessed against the 1% AEP criteria for fluvial flooding and against the 0.5% AEP criteria for tidal flooding. The Environment Agency’s hydraulic models may be made available for use by developers to determine the site’s vulnerability to flooding. The developer will need to firstly ensure that the models are fit for purpose and sufficiently detailed to provide an accurate understanding of flood risk to the site. If existing models are not available, then a developer will need to assess the extent and requirements of any modelling work that is required. Detailed hydraulic modelling will involve the following: • carrying out a hydrological assessment using Flood Estimation Handbook techniques and using gauging records where available; • constructing an in-bank model using up to date survey data including structures, e.g. bridges, weirs, culverts and sluices; • extending the in-bank model to include floodplains where necessary using appropriate hydraulic modelling approaches to replicate the extent, storage and conveyance of the floodplains, e.g. through extended cross sections, reservoir units or 2-D modelling. • calibrating or verifying the hydraulic model where hydrometric monitoring data or flood records are available; • carrying out sensitivity analysis to confirm modelling assumptions and assess climate change impacts; and • mapping of flooding extents.
Defended Floodplain Development sites within a defended tidal or fluvial floodplain are at particular risk due to the risk of the defences being overtopped or breached, resulting in the rapid onset of fast flowing and deep water flooding with little or no warning. Residual risk from the breach or overtopping of defences needs to be considered as part of a FRA. DEFRAs4 Flood Risk Assessment Guidance for New Development provides guidance on the level of risk related to distance and flood depth for overtopping and breaching scenarios. The objectives of a breach analysis are as follows: • to determine the Rapid Inundation Zone where there is a potential risk to life; • to investigate the impact of the proposed development on the flood risk to others; and • to test the effectiveness of mitigation measures. Consideration of flood risk behind defences should take into consideration the standard of protection and design freeboard of the flood defence along with its condition and potential mechanisms of failure. The parameters of a breach in terms of potential location and width as well as the duration of a flood event should be agreed with the Environment Agency prior to any analysis. Raised Floor Levels It may be feasible to reduce the risk to a development through raising the ground level above the design flood level, as shown below:
4 Flood Risk Assessment Guidance for New Development Phase 2: Framework and guidance for Assessing and Managing Flood Risk for New Development – Full Documentation and Tools. R&D Technical Report FD2320/TR2. Defra/Environment Agency 2005
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Flood Protection Level
Freeboard Design Flood Level
Floor levels should be raised above the 1% AEP fluvial flood level plus an allowance for climate change assuming a 20% increase in flow over the next 100 years. There is no need to allow for the 0.5% AEP coastal/tidal flood levels in the Bedford Borough as there is no risk of coastal/tidal flooding. Floor levels will be subject to the approval of Bedford Borough and/or the Environment Agency/IDB. Compensatory Storage Where development is proposed in undefended areas of floodplain, which lie outside of the functional floodplain, the new building footprint and any ground raising will effectively reduce the flood storage capacity of the site. The potential impacts on flood risk elsewhere need to be considered. Raising existing ground levels may reduce the capacity of the floodplain to accommodate floodwater and increase the risk of flooding by either increasing the depth of flooding to existing properties at risk or by extending the floodplain to cover properties normally outside of the floodplain. Flood storage capacity can be maintained by lowering ground levels either within the curtilage of the development or in close proximity elsewhere to provide at least the equivalent volume of storage lost to the development at a nearby location and at the same level. Further guidance on compensatory storage is available within ‘Development and Flood Risk- Guidance for the Construction Industry’, CIRIA 624 section A3.3.10. For development in a defended area of floodplain, the potential impact on residual flood risk to other properties needs to be considered. New development behind flood defences can increase the residual risk of flooding if the flood defences are breached or overtopped by changing the conveyance of the flow paths or by displacing flood water elsewhere. If the potential impact on residual risk is unacceptable then mitigation should be provided. 5.1.6 Surface Water Drainage Assessment Developers should demonstrate that the disposal of surface water from the site will not exacerbate existing flooding from all new development within Flood Zones 3 and 2 and from any development greater than 1Ha in Flood Zone 1 or within areas that are known to suffer from surface water drainage or sewer flooding. A surface water drainage assessment should be undertaken to demonstrate that surface water runoff from the proposed development can be effectively managed without increasing flood risk elsewhere. A surface water drainage assessment should include the following: • Assessment of whether the development will increase the overall discharge from the site by calculating the change in area covered by roofs and hard-standing. • Details of how overland flow from the new development can be intercepted to prevent flooding of adjacent land. • Details of how additional onsite surface water attenuation can be provided to mitigate against known flooding problems or as a result of incapacity on the drainage systems. • Demonstration that overland flows will not increase flood risk to both existing development and receiving watercourses.
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• Calculations showing pre and post development impermeable areas, discharge rates and method of disposal including storage volumes where required. Agreement of these details should be sought from the Environment Agency/IDB and sewerage authorities. Further guidance on Surface Water Drainage methodology, design and implementation is contained within Ref. 17 and ‘The SUDS Manual’ CIRIA 697, 2007. 5.1.7 Selection of Appropriate Mitigation Measures The sequential approach should be applied within development sites to locate the most vulnerable elements of a development in the lowest risk areas. Where vulnerable development cannot be allocated within low risk areas then measures need to be put in place to mitigate against the flood risk. There are several sources of information on potential mitigation measures, as follows: • Flood Risk Assessment Guidance for New Development, Environment Agency R&D (FD2320) • Development and Flood Risk – Guidance for the Construction Industry, CIRIA 624 The Environment Agency R&D Guidance on Flood Risk Assessments for new development suggests that mitigation measures can be split into three types: • Measures that reduce the physical hazard, e.g. through raised defences or flood storage • Measures that reduce the exposure to the hazard, e.g. raise properties above flood levels • Measures that reduce the vulnerability to the hazard e.g. Flood Warning or emergency planning. The selection of appropriate mitigation measures depends on the requirements of the development and its sensitivity to flood risk. Any mitigation measure selected should be sustainable in the future by taking into consideration the impact of climate change on flood risk. The residual risk of developing an area vulnerable to flooding with mitigation measures in place should also be considered. Flood defence walls or embankments Flood defences, fully funded by the development can be constructed and maintained in perpetuity to protect a new development. However, the impact on the risk of flooding elsewhere with defences in place needs to be assessed and managed, for example, through the provision of compensatory storage as detailed in section 5.1.5. Residual risk of flooding with flood defences also needs to be assessed and managed. It should be noted that the construction of new defences to facilitate new development should only be undertaken in exceptional circumstances and alternative options should be considered in the first instance. Flood Storage Flood storage either offline or online can be used to manage water levels at or downstream of a development site. Building Design Flood management measures only manage the risk of flooding rather than remove it completely. Therefore, buildings should be designed to be flood resistant and flood resilient where they are built behind flood defence systems. Flood resistance is the prevention of flood water entering a building through, for example, flood barriers or raising floor levels. Flood resilience is ensuring the finish (e.g. type of flooring) and services (e.g. electrics) are such that following a flood the building can be returned quickly to its normal operation. Guidance on flood resilience is contained within ‘Improving the Flood Performance of New Buildings- Flood Resilient Construction’ DGCL, May 2007 and a basic level of flood resistance and resilience can be achieved through good building practice and complying with Building Regulations (ODPM, 2000).
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Flood Warning The Environment Agency provides Flood Warnings to a number of existing properties at risk of flooding to enable owners to protect life and manage the effect of flooding of their property. Flood Warning should only be provided as a measure to manage residual risk and should not be used as the sole measure to offer protection to a development. Access and Egress PPS25 requires that safe access and escape is available to and from new developments in flood risk areas. Where possible, safe access routes should be located above design flood levels and an evacuation procedure should be in place for an extreme flood event. If safe access cannot be provided for all events then a safe haven of sufficient size to accommodate all occupiers of the development should be provided within the development. 5.2 Site Specific Information for Developers In addition to the development allocations that Bedford Borough Council has already committed to (see Appendix A.2) additional allocations will be made as part of the Bedford development framework. The focus for development will be the urban areas of Bedford and Kempston, but allocations may also be made in the Key Service Centres. This section will provide some information on the location of the Key Service Centres and detail the causes for consideration, in terms of potential flood risks, that face developers wishing to develop in these areas. In general, the Bedford Borough is not at risk from coastal or tidal flooding and therefore all site specific FRAs need only acknowledge this factor. It is also important to note here that although the Key Service Centres have been assessed in terms of potential flood risks, the site specific FRAs will need to investigate all sources of flood risk. 5.2.1 Key Service Centres The Key Service Centres are as follows: • Sharnbrook • Harrold • Bromham • Clapham • Wilstead • Great Barford • Stewartby • Wootton
The following information relates to the settlements themselves rather than the parish. Sharnbrook Sharnbrook is a small village which is at risk of flooding from both the River Great Ouse and the Sharn Brook, however some of the area lies within the Environment Agency’s Flood Warning area. If sites in this area were to be developed a site specific FRAs will be required and may be subject to Exception Testing. Modelling may be required to refine Flood Zones 2 and 3 which are
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based upon the Sharn Brook. There are existing hydraulic models which can be adapted and used to refine the Flood Zones. Harrold A significant portion of the village of Harrold is at risk of flooding from both the River Great Ouse and smaller drains. There is the potential for flooding from surface water bodies in the area. Harrold lies within the Environment Agency’s Flood Warning Area and those residents that are at risk of flooding from the River Great Ouse are warned of potential flooding. If sites in this area were to be developed a site specific FRA will be required and may be subject to Exception Testing. Modelling may be required to refine Flood Zones 2 and 3. There are existing hydraulic models which can be adapted and used to refine the Flood Zones. Bromham Bromham lies on the banks of the River Great Ouse and also several other smaller watercourses. If sites in this area were to be developed further a site specific FRA will be required and may be subject to Exception Testing. Modelling may be required to refine Flood Zones 2 and 3. There are existing hydraulic models of the River Great Ouse which can be adapted and used to refine the Flood Zones. The smaller watercourses will require models to be developed which will refine the flood risk in these areas. Clapham Clapham is a small village that lies on the banks of the River Great Ouse, although only a small part of the village lies within Flood Zones 2 and 3. If development were to take place in Clapham it would need to be outside the floodplain, otherwise it would be subject to a site specific FRA (unless in excess of 1 hectare) and a potential Exception Test. As previously stated there is an existing hydraulic model of the River Great Ouse which could be adapted to refine the Flood Zones if necessary. Wilstead Wilstead is a small village that lies to the south of Bedford and adjacent to the new Wixams development. Although Wilstead itself is not subject to flooding from any main watercourses, it does have a number of ordinary watercourses that cross it. The ordinary watercourses are the responsibility of the IDB and the IDB have advised that the area is vulnerable to flooding from these ordinary watercourses. There is also a flood storage reservoir which reduces the risks of flooding to the existing properties (although operational issues have arisen due to the presence of protected species). Should development be required in the village, the ordinary watercourses will need to be modelled. At present the only hydraulic models that the IDB hold are for the Elstow Brook, which was developed for the Wixams allocation. The hydraulic model outlines have been developed by Hannah Reed and Associates and will be available for the Level 2 SFRA. Great Barford The village of Great Barford lies on the banks of the River Great Ouse. Smaller ordinary watercourses have their confluence with the River Great Ouse at Great Barford, and the IDB have advised that the area is vulnerable to flooding from the smaller ordinary watercourses. There are also two flood storage reservoirs at Great Barford, which reduces the risks of flooding to existing properties. If further development were to be required in this area, the development would need to take place outside of Flood Zones 2 and 3 or they would be subject to a site specific FRA and potentially an Exception Test, depending on the type of development. Stewartby The majority of Stewartby lies within Flood Zone 1, however if a development proposal is larger than 1Ha, the developer will be required to produce a FRA and an appropriate surface water assessment will be required. Also if development is proposed outside of the current settlement area, it may be within Flood Zones 2 and/or 3 and as such a site specific FRAs may therefore be required.
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Wootton Wootton lies within Flood Zone 1 and is not at risk of fluvial flooding. Any development in this area will have to mitigate surface waters and minimise flood risks from pluvial events and if a development is larger than 1Ha, the developer will be required to produce a FRA and an appropriate surface water assessment will be required. Also if development is proposed outside of the current settlement area, it may be within Flood Zones 2 and/or 3 and as such a site specific FRA may therefore be required.
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6. Conclusions and Recommendations 6.1 Conclusions 1. This Level 1 SFRA report provides an overview of the planning context in relation to flood risk and development within the Bedford Borough. Flood risk is considered within each of the tiers of planning policy; nationally within the Planning Policy Statements, regionally within the RSS and locally within the Local Plan and the new Bedford Development Framework. 2. Data has been collected through consultation with Bedford Borough Council, the Environment Agency, the Bedford Group of Drainage Boards, and Anglian Water. The data collected has provided information on all sources of flood risk, flood defences, Flood Warning, land allocation and the geology and topography of Bedford Borough. 3. The primary source of flood risk within Bedford Borough is fluvial flooding. The River Great Ouse runs from west to east through the middle of Bedford and is the greatest source of flood risk. In addition to this there are smaller main rivers in the north of the Borough. There are also IDB maintained watercourses that cross the catchment (mainly on the south-eastern side) which could cause a potential flood risk. 4. Parts of the Borough are at risk of flooding from surface and foul water sewers. 5. There is only one record of groundwater flooding in the Bedford Borough, identified in Ref: 12 as the village of Keysoe and was affected by groundwater flooding in 2000-2001. Although the groundwater flood risk is perceived to be low, there are numerous old mineral workings within the Borough which, if altered and re-developed, may give rise to a change in the movement of groundwater flow. There are also some areas which have been identified by the IDB as lying very wet. 6. There are no formal flood defences along the River Great Ouse, with the exception of Bedford Town Centre. The Environment Agency own and maintain many of the defences along the main rivers. The IDB are also responsible for maintaining flood defences on their own watercourses. 7. The Environment Agency currently operates a Flood Warning service in for areas indicated within Appendix A.5, however only 25-30% of properties currently receive Flood Warnings in the Bedford Borough. In addition to this they also offer early alerts of possible flooding in the form of Flood Watches. 8. Development that takes place in ‘Functional Floodplain’ (assumed to be Flood Zone 3 as part of this Level 1 SFRA), or areas highlighted as at risk from other sources of flooding (e.g. Groundwater Flooding) that are in close proximity to existing development and/or restrictive watercourses will potentially increase flood risk. It is these areas, where development should be avoided in the first instance. 9. Climate change is expected to increase the flood risk to some properties within Bedford Borough. 10. The Bedford Borough has the potential to include SUDS within future developments. Existing projects have incorporated lagoons and ponds and the nature of the catchment would dictate this to be the most efficient form of SUDS. It is the aspiration of the Surface Waters Group to utilise the disused brick pits within the area for flood storage and recharge. Bedford Borough Council wishes to adopt a strategic approach to the management of surface water within the Bedford Borough. 11. Any additional hydraulic modelling that is carried out as part of the Level 2 Bedford SFRA, should utilise existing hydraulic models, however these models will need to be updated to include more detail. In places where there are no hydraulic models available, hydraulic models may need to be developed.
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6.2 Recommendations for the Level 2 SFRA These recommendations, along with any other requirements identified by the Bedford Borough Council, should feed into a more detailed proposal and methodology for the Level 2 SFRA. 1. The Level 2 SFRA should build on this Level 1 SFRA by providing advice on flood defence and mitigation measures for the Key Service Centres and the Growth Area identified in the Council’s Core Strategy & Rural Issues Plan. 2. Further work should be undertaken to refine the extent of flood risk. This should be done by refining the Flood Zones based on existing hydraulic modelling and, if necessary, further modelling will be carried out. This will aid the identification of Flood Zones 3a and 3b and the flood outline for climate change. A table of the recommended modelling is presented below;
Existing/New model Watercourse Modelling Requirement
Wilden IDB Drain IDB currently undertaking survey. Potential to build model to define flood extents
EA ARTS2C model Great Ouse Validate models with recent data, derive extreme event hydrology. Simulate extreme River Kym events to define Flood Zones. Risely Brook
Elstow Brook Elstow Brook Simulate extreme flood events to define Flood Zones Table 6.1 – Recommended further hydraulic modelling
3. Detailed assessment of the flood risk within the Key Service Centres should be undertaken in consultation with Bedford Borough Council. 4. A detailed assessment of the proposals for the Milton Keynes to Bedford Canal should be carried out to determine the potential extent of flood risk that may arise as a consequence of its development. 5. It is recommended that the villages of Keysoe, Wilstead, Cotton End and Cardington are assessed further to determine if there is any potential flood risk from groundwater flooding. 6. It is recommended that the Level 2 SFRA should look at the effects of groundwater movement in the area as the change in use of these may affect the flow of groundwater which could potentially give rise to flooding. 7. Development control policies should be recommended for both discharging surface water and flood risk for different types of catchments, land uses and locations. Information contained within a surface water management strategy should include whether it is ‘brownfield’ or ‘greenfield’ or urban or rural. 8. Opportunities for restoring the natural floodplain should be identified, as well as potential opportunities to remove development from areas that flood at present and relocate to lower flood risk areas. 9. Where flood defences have a standard of protection lower than 1% AEP for fluvial flood risk, options to raise these defences to the required standard should be identified. 10. The guidance for developers should be developed further in order to provide a tool for Bedford Borough Council to assist with planning approval procedures. A checklist of technical analysis relating to each source of flood risk should also be provided. The analyses should be undertaken by developers for site specific Flood Risk Assessments.
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11. Flood Risk Factsheets should also be prepared for each of the Key Service Centres. These Factsheets should be aimed at planners, engineers and members of the public who require a ‘quick reference’ to the flood risk for an area.
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References
1. Department for Communities and Local Government, 2006. Planning Policy Statement 25: Development and Flood Risk. 2. Department for Communities and Local Government, 2007. Development and Flood Risk; A Practice Guide Companion to PPS25. 3. WSP, 2006. Bedford Town Centre SFRA. 4. Bedford Borough Council, 2002. Updated Sept. 2007. Bedford Borough Local Plan. 5. Minutes of Meeting held on 11th January 2008, Atkins Ref: 5061078/41/DG/018. 6. HR Wallingford, 1993. River Ouse at Bedford Flood Alleviation Study. 7. Atkins, 2003. Environment Agency Anglian Region ARTS2c Region-wide Modelling Project: Bedford Ouse Catchment Block. Data Review. 8. Atkins, 2003. Environment Agency Anglian Region ARTS2c Region-wide Modelling Project: Bedford Ouse Catchment Block. Catchment and Network Conceptualisation. 9. Atkins, 2003. Environment Agency Anglian Region ARTS2c Region-wide Modelling Project: Bedford Ouse Catchment Block. Model Solution. 10. Atkins, 2004. Environment Agency Anglian Region ARTS2c Region-wide Modelling Project: Bedford Ouse Catchment Block. Model Setup. 11. Environment Agency, 2007. Great Ouse Catchment Flood Management Plan: Consultation Draft Plan. 12. DEFRA, March 2004, Strategy for Flood and Coastal Risk Management: Groundwater Flooding scoping study (LDS23). 13. The Surface Waters Plan, June 2002, Marston Vale Surface Waters Group. 14. Bedford Borough Council Development Plan Document: Core Strategy and Rural Issues Plan Submission Version; July 2006; Bedford Borough Council. 15. East of England Plan Draft Revision to the Regional Spatial Strategy for the East of England; December 2004; East of England Regional Assembly. 16. Review of April 1998 Hydrology – Bedford Ouse Catchment; Atkins, 2003. 17. Interim Code of Practice for Sustainable Drainage Systems; National SUDS Working Group; CIRIA; July 2004.
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